Here is a solution for controlling the mplayer using nodejs. Much cleaner and more efficient than my fifo-solution. Instead of process.stdin i could use a socket or an webservice or whatever, very simple.

node_controlled_mplayer.js

var cp = require("child_process");
var fs = require("fs");

player = cp.spawn("mplayer",
  ["-slave","-idle","-quiet"]
);
player.stdout.on("data",function(data){
  console.log("" + data);
});
player.stderr.on("data", function(data){
  console.error("" + data);
});

process.stdin.setEncoding('utf8');
process.stdin.on('readable', function(){
  var chunk = process.stdin.read();
  if (chunk !== null){
    player.stdin.write(chunk);
  }
});